Bug#802919: unison: synchronization incompatibility when built with Ocaml versions pre/post-4.02

[Conrad Hughes <debbugs@xrad.org>]

Hi Stéphane,

Thanks for your response.

By this:

> > Possibly some pressure should be put on upstream to (1) catch the
> > relevant exception and flag the ocaml version dependency,

I meant that since this is such a non-obvious failure, which should
never happen normally (I've never seen a unison data stream corrupted in
probably fifteen years of using it), that it might be reasonable and
helpful to catch the exception caused by the serialisation change and to
add a message saying "This exception may be because the two versions of
unison were compiled with different versions of OCaml", with a reference
to some relevant documentation (e.g. bug report or release notes).

--

I'm unfamiliar with how the Debian release process works internally, and
minimising the work involved in packaging unison is of course essential
— you're already saving all Debian users a huge amount of work by
packaging it, so thank you for that.

At the moment because you're building with ocaml-4.01, the published
unison is backwards compatible but not forwards compatible; can you
continue to build unison with 4.01 now that Debian's on ocaml-4.02, or
is that going to become difficult as well?

My initial thought process was that producing two conflicting ocaml
builds, say ocaml_pre_402-4.01.0 and ocaml-4.04.2 (or whatever the
actual versions are), which could *not* be installed simultaneously,
would have been the easiest route to automatically building multiple
versions of unison on some kind of automated build farm (including the
ocaml version in unison package names), but that's probably because I
don't understand what the actual release process is like.

So if two ocaml versions is not a practical way of proceeding, do you
think there's any reasonable way of producing pre- and post-402 releases
of unison for Debian in the future?

If not, then would it be possible to include a release note (the kind
that you get shown and emailed when you upgrade) warning users about the
incompatibility?  I realise that arguably this note should not yet have
been sent since Debian's unison is still backwards compatible; perhaps
you already have it planned for the -4.02 changeover..

Regards,
Conrad